Place watermelon in container of electric blender, and process until smooth.
Reserve 2 1/4 cups puree, discarding any remaining puree.
Combine gelatine and 1/4 cup watermelon puree. Bring remaining watermelon puree
to a boil in a small saucepan. Remove from heat; add gelatine mixture, and stir
until gelatine dissolves. Stir in sugar and lemon juice, and chill mixture until
consistency of unbeaten egg white.
Fold whipped cream into chilled watermelon mixture. Spoon into a lightly oiled
Cover and freeze 8 hours or until firm; unmold onto serving plate.